The invention relates to a device for determining the orientation of a vehicle having an at least partly ferromagnetic body, comprising a magnetic field sensor which is arranged in the vicinity of an outer wall of the body and which is rigidly connected to said outer wall.
An example of such a device is known from EP-B-0 067 337. The known device is based on the assumption that the soft magnetic parts of the metal body of the vehicle bundle the lines of force of the terrestrial magnetic field and hence contribute to more accurate measurement of this field. In this respect it is assumed that the effect of disturbing magnetic fields caused by hard magnetic parts of the vehicle can be compensated for by way of a suitable correction method. For an example of such a correction method, the cited document refers to EP-A-0 067 338 corresponding to U.S. Pat. No. 4,416,067. In accordance with said method, the vehicle must be driven around in a circle, measurements then being performed to enable calculation of correction factors. Such correction methods are comparatively satisfactory when the magnetization of the parts of the body causing the disturbance is substantially constant. In that case the comparatively complex and time-consuming correction method need be performed only once, for example prior to the beginning of a ride in the vehicle.
However, it has been found in practice that the remanent magnetism present in parts of the body can change comparatively easily. The magnetization of such parts of the body, therefore, changes comparatively often and in an unpredictable manner. This takes place, for example when the vehicle enters an external magnetic field. Such magnetic fields occur in the vicinity of large steel constructions such as bridges, freight trucks or buildings involving a large amount of structural steel. Large magnetic fields also occur in the vicinity of electric leads carrying large direct currents, for example, current leads for trains or trams. For example, currents up to 4000 A can flow through the overhead wires of railroads, giving rise to magnetic fields having a horizontal component of 4.4.times.10.sup.-4 T. For the purpose of comparison it is to be noted that the terrestrial magnetic field in the Netherlands has a horizontal component of 2.7.times.10.sup.-5 T. Under the influence of such external magnetic fields, the magnetization of ferromagnetic pans of the body changes continuously. On the other hand, a partial demagnetization of such parts can occur, for example due to shocks and temperature cycles and magnetic alternating fields which occur, for example in the vicinity of high-voltage cables.